Asphalt is commonly used in the construction and paving of roads. Asphalt is a mixture of aggregate material, such as sand, gravel, and crushed stone, with hot bitumen. The bitumen coats the aggregate material to give the asphalt, which may be spread as a uniform layer upon a road bed and compacted and smoothed with heavy rolling equipment.
Asphalt invariably contains sulfur. The amount of sulfur will depend on the origin of the crude oil, as well as the processes used to refine the crude oil, into asphalt. The sulfur may exist in different forms. For example, sulfur may be in the form of hydrogen sulfide. Hydrogen sulfide, or dihydrogen sulfide, is a chemical compound with the formula H2S. It is a colorless, poisonous, flammable gas with the characteristically regarded foul odor.
Hydrogen sulfide may be released form asphalt, in particular when the asphalt is heated to a certain temperature. For example, hydrogen sulfide results from the dehydrogenation reactions that occur between bitumen and sulfur at the hot mixing temperatures, for example temperatures greater than 140° C. Hydrogen sulfide emissions are regulated. Therefore, there exists a need to reduce the amount of hydrogen sulfide in asphalt.
A variety of polymers are used to modify asphalt. The degree to which a polymer improves an asphalt's properties depends on the compatibility of the polymer and the asphalt; e.g., a polymer that does not separate in a mixture of asphalt and polymer during storage. Highly compatible or compatibilized polymers are more effective in providing property improvements. An extensive range of additives have been used for the purpose of “crosslinking” polymers and asphalts, thereby rendering the mixture compatible. For example, sulfur is a well-known crosslinking agent.
Polyphosphoric acid (Hn+2 PnO3n+1) is a polymer of orthophosphoric acid (H3PO4). Polyphosphoric acid offered commercially is a mixture of orthophosphoric acid with pyrophosphoric acid, triphosphoric and higher acids. Superphosphoric acid is a similar mixture sold at 105% H3PO4. Other grades of phosphoric acid may contain water, but are not typically used in asphalt modification. This eliminates issues of foaming and corrosion at the refinery or terminal. PPA's major applications are surfactant production, water treatment, pharmaceutical synthesis, pigment production, flame proofing, metals finishing and asphalt modification. This circular will specifically discuss the use of PPA as an asphalt modification.
PPA has been widely used in refineries to modify the performance properties of asphalt. Due to its strong acidity, however, the PPA would revert most H2S scavengers and allow undesirable release the hydrogen sulfide from scavenger treated asphalt.
Accordingly, the present disclosure provides for a reduced or low release of hydrogen sulfide during the preparation of asphalt, as well as in the final asphalt material.